1. Field of the Invention
The present invention relates to a memory device using thin film transistors and a memory circuit employing the same.
2. Description of the Related Art
Recently, memory elements constituted by thin film transistors have been developed as memory elements for a memory device such as an EEPROM capable of electrically performing write, erase, and read operations.
Such a memory element is disclosed in, e.g., U.S. Ser. No. 427,252.
According to U.S. Ser. No. 427,252, one memory element is constituted by a memory transistor having a memory function and a selection transistor having no memory function, which are connected in series on an insulating substrate.
By ON/OFF-controlling the selection transistor, the corresponding memory transistor is selected, and data write, erase, or read operation is performed with respect to the selected memory transistor.
In this memory element, the memory and selection transistors consist of thin film transistors. Therefore, such memory elements can be easily manufactured at low manufacturing cost as compared with memory elements of widely used EEPROMs.
With the recent improvement in performance of electrical equipment and the like, however, memory devices used for such equipment are required to have a higher packing density at lower cost.